This invention relates to a wire electrode feeding apparatus for a wire electrodischarge machine.
Wire electrodischarge machines are widely used to cut metal by electrical discharge between a wire electrode and a workpiece. To commence a cutting operation, the wire electrode must be inserted through a starting hole in the workpiece and through wire guides disposed on either side of the starting hole. Insertion is normally performed automatically, using a motor-driven capstan and pinch roller to feed the wire from a bobbin.
To keep the wire electrode from unwinding from the bobbin and becoming snarled, a pre-tension motor applies a torque to the bobbin that pulls the wire electrode backward, in the direction opposite to the feeding direction. Wire feeding is also opposed by friction between the wire electrode and wire guides. The pinch roller must press the wire electrode against the capstan roller with sufficient force to overcome the pre-tension and friction forces; otherwise the wire electrode will slip on the capstan roller and fail to feed.
If the pinch roller presses too strongly, however, it may flatten the wire electrode, causing additional friction when the wire electrode passes through the wire guides. In the worst case, friction prevents the passage of the wire electrode and the wire electrode becomes snarled at a point between the capstan roller and wire guides. Automatic feeding is then completely disabled; human intervention is required to recover normal operation.
The ideal pressure that should be exerted by the pinch roller depends on the diameter of the wire electrode, and the pre-tension should be adjusted according to the pinch-roller pressure and wire diameter. Prior-art wire electrode feeding apparatus, however, has lacked means of adjusting the pinch-roller pressure and pre-tension, so the problems of slipping and snarling described above have frequently occurred in practice.